The present invention relates to a novel component protein of tight junctions (hereinafter referred to as TJs) of exocrine glands and uses of the protein.
In multicellular animals, information on junction of adjacent cells is deeply related to control and maintenance of life phenomena such as proliferation and differentiation of cells, inflammation, and metastasis of cancers. In many cases, intercellular adhesion molecules that participate in adhesion gather on the surface of cells to form a membrane region uniquely differentiated for adhesion. In particular, in epithelial cells, it has been known that intercellular adhesion molecules such as cadherin are strongly bound to cytoskeleton in the cytoplasm domain.
Such a membrane region, which is called an intercellular adhesion mechanism, is generally classified into four types: gap junction (GJ), adherens junction (AJ), desmosome and tight junction (TJ).
TJ is one of intercellular adhesion mechanisms in the epithelial or endothelial cell layer. It plays a role of a physical barrier for preventing free passage of solutes and water through extracellular space (barrier function), and constitutes a continuous peripheral seal surrounding the cells. Also, it is considered that TJ plays a role of a boundary between apical and basolateral cytomembrane regions for forming and maintaining cell polarity (fence function).
TJs comprise transmembrane protein molecules such as claudin, occludin and JAM and peripheral membrane proteins, such as ZO-1, -2, and -3, cingulin, 7H6, symplekin, Rab3B, Sec6/Sec8 homolog, ASIP/PAR-3, PAR-6, and MAGI-1. Claudin and occludin constitute the backbone of TJ strands and are involved in the barrier function of TJs.
JAM is involved in cell-cell adhesion and/or junctional assembly of endothelial and epithelial cells, as well as infiltration of monocytes through interstices between endothelial cells induced by chemokines. ZO-1, -2 and -3 are scaffold proteins containing PDZ domains and directly bind to claudin and occludin at the cytoplasmic surface of TJ strands. ZO-1, -2 and -3 also bind to F-actin and might regulate TJ functions via cross-linking 1.1 strands and the actin cytoskeleton. Several other PDZ domains—containing proteins localized at TJs might also serve as landmarks to recruit cytoskeletal and signaling proteins to TJ strands. As a non-F-actin binding scaffold protein, MAGI-1/2/3 localizes at TJs and interacts with signaling molecules such as a tumor suppressor gene product, PTEN, and a GDP/GTP exchange protein for Rap small G protein. ASIP/PAR-3 and PAR-6 are cell polarity-related molecules containing PDZ domains and interact with a typical protein kinase C (PKC). ASIP/PAR-3 interacts with JAM. Among peripheral membrane proteins devoid of PDZ domains at TJ strands, Rab3B and Sec6/Sec8 homologs are involved in vesicular transport. Furthermore, cingulin, 7H6 antigen, and symplekin have been known to localize at TJs. However, their functions have not been elucidated yet. Cingulin interacts with ZO-1, -2 and -3, occludin, AF-6, and JAM.
To further characterize the molecular organization of TJ, identification and provision of novel TJ-constituting proteins has been desired.
Meanwhile, a mouse cDNA clone (GenBank accession No. BAB30287) has been reported by RIKEN as a gene whose function is unknown.